a) Field of the Invention
The invention is directed to an optical system for a fundus camera which serves to image the ocular fundus. When imaging the ocular fundus with a camera of this type, flare generally results at the cornea and at surfaces of the optical system and has a degrading effect on the image quality.
In its basic construction, a fundus camera comprises a multiple-stage optical system. An opthalmoscope lens generates an intermediate image that is imaged by a sequential system (principal objective) on a film or a CCD matrix. The opthalmoscope lens is also a component part of the illumination.
b) Description of the Related Art
A particular problem in observing and photographing the fundus is posed by flare at the cornea and at the surfaces of the opthalmoscope lens because the light which is reflected by the retina and which carries the relevant image information is substantially less intense than the light that is reflected before entering the eye. Troublesome corneal flare is normally prevented by dividing the pupil of the eye. For this purpose, the opthalmoscope lens images an illumination ring in the pupil of the eye. The illumination rays reflected at the cornea miss the observation aperture. Only the area within the illumination ring is used for observation.
There are substantially two known concepts for suppressing flare at the opthalmoscope lens.
DE-OS 35 19 442 describes an optical system in which components of light that could enter the observation aperture by reflection at the opthalmoscope lens or cornea are masked out by “black-dot plates” which are arranged at a suitable location in the beam path and which are coated in a defined manner with light-absorbing layers. This method of suppressing reflections has come to be known as “anti-flare dot objective”.
The proximity of the anti-flare dot to the field diaphragm is a disadvantage in this design. The absorption of individual light components can become visible as an inhomogeneous illumination of the ocular fundus. Ring-shaped shadows occur which degrade the image and accordingly impede evaluation by the ophthalmologist.
Another solution is described in U.S. Pat. No. 4,730,910. This solution does away with masking of determined light components within the illumination optics. Instead of the opthalmoscope lens, a multiple-lens objective is used whose lenses are tilted relative to one another in such a way that the direct flares at the glass-air interfaces do not enter the observation aperture. For this purpose, the optical axes of the lenses lie in a plane together with the optical axis of the observation beam path. This solution requires a considerable expenditure on mechanical mounting and has considerable problems with correcting imaging errors.
A system of the type mentioned above exhibits a clear difference between imaging scales in the meridional and sagittal section. Anamorphotic effects can be observed in object imaging as well as in pupil imaging (illumination). The images are distorted and there is no similarity of imaging. Further, the system exhibits coma and astigmatism when imaging the axial point and the imaging errors in the field are not symmetric with respect to rotation.